∫((3x^2)+1)cosxdx
=∫3x^2.dsinx + sinx
=3x^2.sinx - 6∫xsinxdx + sinx
=3x^2.sinx + 6∫xdcosx + sinx
=3x^2.sinx + 6xcosx -6∫cosxdx + sinx
=3x^2.sinx + 6xcosx -5sinx + C